The invention described herein relates generally to diffraction gratings, and more particularly to X-ray to VUV reflection diffraction gratings.
A diffraction grating may be very broadly defined as any arrangement which imposes on an incident wave a periodic variation of amplitude or phase, or both. See the standard text "Principles of Optics, Third Edition", by Max Born and Emil Wolf, Pergamon Press (1965), which is incorporated by reference herein. There are many different kinds of diffraction gratings. Any structure which is periodic in space will serve as a diffraction grating. A very well known type of grating is the transmission diffraction grating, made by ruling equally spaced lines through a silver film that is deposited upon a transparent glass plate. As light passes through this grating, it is diffracted by the narrow slits, usually of equal width and separated by equal distances, in the opaque silver film.
A reflection diffraction grating is produced by ruling a multitude of fine, parallel, equidistant grooves, by means of a diamond point driven by a ruling machine, upon a plate of polished speculum metal. Speculum metal is a hard and brittle alloy of copper and tin, that is capable of taking a brilliant polish, and that is commonly used for making reflectors. If light is incident upon the grating at any angle with respect to the normal, light will be diffracted from the surface of the grating at all angles. A non-spectral, central image is emitted from the grating in the direction of regular reflection, where the angle of incidence is equal to the angle of diffraction. Two series of spectral images, of increasing order, are laterally disposed on either side of the central, zero order image. The dispersion, or separation, of the wavelengths within a diffracted spectrum, increases in proportion to the order of the spectrum. The shape of the grooves of the grating determines the direction into which the diffracted light will be predominantly thrown. For example, if one face of the grooves is flat, a maximum of energy will be cast into the direction which makes an angle with this face which is equal to the angle made by the incident light with this face. This technique is called blazing, and the spectral order that lies in the blazed direction will be intense.
Synthetic structures, known as multilayers and consisting of alternating layers of high and low atomic number elements, are described in "Multilayers for X-ray Optics", Opt. Eng. 25, pages 898 to 915 (1986), by Troy W. Barbee, Jr. A molybdenum-silicon multilayer monochrometer for use in the extreme ultraviolet, is described by Troy W. Barbee, Jr. et al in Appl. Phys. Lett. 50 (25), pages 1841 to 1843 (1987).
Keski-Kuha, in Applied Optics 23, 3534 (1984), discloses the potential use of layered synthetic microstructures as coatings on diffraction gratings to enhance normal incidence reflection efficiencies. An actual 5000 line per millimeter plane holographic grating was produced that was first coated with iridium, and then with a five layered iridium-silicon layered synthetic microstructure. At 304 Angstroms, the first order efficiency of the grating was enhanced by approximately the factor three. At longer wavelengths the efficiency was reduced.
Vidal et al, in SPIE Vol. 563, 142 (1985) profess to develop a formalism for rigorously computing the efficiency of multilayer coated gratings.
Ceglio et al, in SPIE Vol. 563, 360 (1985) discuss a concept for output coupling from an X-ray laser cavity, wherein a diffraction pattern is lithographically produced on or in a multilayer structure. It is suggested that the pattern may provide periodic or aperiodic amplitude or phase modulation of mirror reflection, with reflected X-rays diffracted into multiple orders. Note particularly FIG. 4(c).
Ciarlo et al, in SPIE Vol. 688, 163 (1986), disclose the use of anisotropic etching to fabricate diffraction gratings on silicon wafers for use as substrates for layered synthetic multilayers. Work on the fabrication of blazed gratings is reported.
Franks in U.S. Pat. No. 3,980,883 issued Sept. 14, 1976 discloses an X-ray diffraction grating in which the grooves are uniformly spaced but of varying depth, to thereby reduce the variation of efficiency of the grating in the region of the wavelength of maximum efficiency. The maximum efficiency of the grating is lowered. To function, X-rays must impinge on this phase grating at extremely small angles of grazing incidence.
Keem et al in U.S. Pat. No. 4,693,933 issued Sept. 15, 1987 teach X-ray dispersive and reflective structures that comprise alternating layers of metallic and non-metallic materials, with the potential of the layers interacting controlled by utilizing an interfacial buffer layer between the layers.
Wood et al in U.S. Pat. No. 4,675,889 issued June 23, 1987 discuss X-ray dispersive structures, comprised of layer sets formed on one another, which reflect two or more wavelengths at the same or different angles.
Yano et al in U.S. Pat. No. 4,313,648 issued Feb. 2, 1982 disclose a patterned multi-layer structure for a stripe filter used for a photoelectric pickup tube. The multi-layer optical filter is patterned by reactive sputter etching into a stripe pattern.
Spiller in U.S. Pat. No. 3,887,261 issued June 3, 1975 teaches a reflective structure for optical waves that comprises an array of alternate layers of high and low absorbing elements, with the layers coated on top of each other.
Gamble in U.S. Pat. No. 3,688,109 issued Aug. 29, 1972 discusses the use of structurally layered heavy metal chalcogenides as diffraction grating crystals in X-ray optical assemblies.
Daxinger in U.S. Pat. No. 4,101,200 issued July 18, 1978 discloses a light transmitting coating comprised of alternating metallic and non-metallic layers.
Gale et al in U.S. Pat. No. 4,576,439 issued Mar. 18, 1986 teach an improved authenticating device wherein a reflective and diffractive coating layer is situated between the substrate and overcoat layers of the device. The coating layer is divided into a set of small and slightly separated regions, thereby allowing a direct bond of the overcoat layer to the substrate layer within the separation areas.
Dumond in U.S. Pat. No. 2,688,094 issued Aug. 31, 1954 discloses a point focusing X-ray monochromator for low angle X-ray diffraction, that utilizes two crystal reflecting surfaces.
Because of their great power for penetration, X-rays are diffusely scattered from within the body of any material, and consequently are essentially not specularly reflected from any single surface, when directed at that surface at ordinary angles of incidence such as, for example, 30 or 45 degrees. Even though X-rays are reflected from polished surfaces when introduced thereto at nearly grazing incidence, and X-ray diffraction gratings operative at extremely shallow angles of incidence, measured in minutes of arc, have been constructed, there remains a critical need for high resolution and high efficiency reflection diffraction gratings, for X-ray to VUV energies, that are operative at normal and near normal angles of incidence.